Fixture for Shape-Sensing Optical Fiber in a Kinematic Chain

ABSTRACT

Localized strain is effectively eliminated in a shape sensing optical fiber where the fiber exits a link in a kinematic chain. In one aspect, the fiber is positioned in a channel within a fiber fixture portion of the link, and a surface of a lip of the channel is curved. The curved surface of the lip begins tangent to a wall of the channel and has a maximum radius of curvature that is less than the minimum bend radius that the fiber will experience during shape sensing. In another aspect, the fiber is positioned within a shape memory alloy tube that extends from the link.

BACKGROUND

1. Field of Invention

The invention relates to fiber optic sensors, and more particularly tofiber optic sensors for detecting the fiber's position and orientation,and still more particularly to fixtures used to constrain a fiber opticsensor with reference to a link in a kinematic chain.

2. Art.

In shape sensing systems that use interferometric techniques tointerrogate an optical glass fiber with refractive index change (Bragg)gratings, the physical resources of the interrogating hardware limit thenumber of data points that can be used to describe the fiber's shape atany particular time. Specifically, the product of the capacity of theinterrogator's electronic data processor and the interrogating laser'sfrequency sweep range is approximately proportional to the product ofthe length of the fiber being sensed and the peak strain that occurswithin the sensed length. Bending is typically the primary source ofstrain in an optical fiber used as a shape sensor. The peak straineffectively occurs at the point of minimum bend radius in the sensedportion of the fiber.

In order to use a fiber optic shape sensor in a kinematic chain, such asa robot manipulator arm, it is often useful to constrain one or moreportions of the sensed length in known positions and/or orientationsrelative to the individual links in the chain. Known ways of holding anoptical fiber in place include the use of collets and various othermechanical clamping mechanisms, as well as gluing, etc. When applied toa moveable joint, however, many of these methods of holding a fiberresult in a point load being applied to a location along the fiber,which results in exceedingly large peak strains at that location. Whatis required is a way to effectively eliminate these large peak strains.

SUMMARY

In a kinematic chain that includes a first link, a second link, and ajoint that couples the first and second links, an optical fiber extendsbetween the first and second links across the joint. The optical fiberis configured with fiber Bragg gratings for shape sensing.

In one aspect, the fiber bends as the joint moves, and therefore aminimum bend radius for the fiber is defined when the joint reaches alimit in its range of motion. A fixture is associated with the firstlink and constrains the fiber with reference to the first link. Thefiber is positioned within a channel in the fixture, and a length offiber Bragg gratings in the fiber is positioned adjacent a lip of thechannel. A surface of the lip is curved in one or more of the planes inwhich the fiber may bend due to the joint's movement. The curved surfacebegins tangent to a wall of the channel, and the maximum radius ofcurvature of the curved surface that may contact the fiber is less thanthe minimum bend radius of the fiber that has been defined by thejoint's range of motion. The curved surface of the lip effectivelyeliminates the localized strain in the shape sensing optical fiber wherethe fiber exits the link in the kinematic chain.

In another aspect, the shape-sensing optical fiber is positioned withina shape memory alloy tube that extends between the first and secondlinks in the kinematic chain. The optical fiber is positioned such thatthe length of fiber Bragg gratings in the fiber is adjacent the locationwhere the shape memory alloy tube extends from the first link.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a portion of a kinematic chain.

FIG. 2A is a diagrammatic cross-sectional view of a fixture portion of alink in a kinematic chain.

FIG. 2B is a diagrammatic cross-sectional view of a shape-sensingoptical fiber mounted in a fixture portion of a link in a kinematicchain.

FIG. 2C is a diagrammatic cross-sectional view of another assembly for ashape-sensing optical fiber mounted in a fixture portion of a link in akinematic chain.

FIG. 3A is a cross-sectional diagrammatic view of another optical fiberfixture portion of a link in a kinematic chain.

FIG. 3B is a cross-sectional diagrammatic view of a shape-sensingoptical fiber mounted in a fixture portion of a link in a kinematicchain.

FIG. 4 is another cross-sectional diagrammatic view of a shape-sensingoptical fiber mounted in a fixture portion of a link in a kinematicchain.

FIG. 5 is yet another cross-sectional diagrammatic view of ashape-sensing optical fiber mounted in a fixture portion of a link in akinematic chain.

FIG. 6 is still another cross-sectional diagrammatic view of ashape-sensing optical fiber mounted in a fixture portion of a link in akinematic chain.

FIGS. 7A-7D are diagrammatic views that illustrate various ways that afixture portion may be positioned with reference to a link.

FIG. 8 is a diagrammatic front elevation view of a test apparatus.

FIG. 9 is a diagrammatic view of a minimally invasive surgicalinstrument.

DETAILED DESCRIPTION

This description and the accompanying drawings that illustrate aspects,implementions, and embodiments of the present invention should not betaken as limiting—the claims define the protected invention. Variousmechanical, compositional, structural, electrical, and operationalchanges may be made without departing from the spirit and scope of thisdescription and the claims. In some instances, well-known circuits,structures, and techniques have not been shown or described in detail inorder not to obscure the invention. Like numbers in two or more figuresrepresent the same or similar elements.

Further, this description's terminology is not intended to limit theinvention. For example, spatially relative terms—such as “beneath”,“below”, “lower”, “above”, “upper”, “proximal”, “distal”, and thelike—may be used to describe one element's or feature's relationship toanother element or feature as illustrated in the figures. Thesespatially relative terms are intended to encompass different positionsand orientations of the device in use or operation in addition to theposition and orientation shown in the figures. For example, if thedevice in the figures is turned over, elements described as “below” or“beneath” other elements or features would then be “above” or “over” theother elements or features. Thus, the exemplary term “below” canencompass both positions and orientations of above and below. The devicemay be otherwise oriented (rotated 90 degrees or at other orientations)and the spatially relative descriptors used herein interpretedaccordingly. Likewise, descriptions of movement along and around variousaxes includes various special device positions and orientations.

In addition, the singular forms “a”, “an”, and “the” are intended toinclude the plural forms as well, unless the context indicatesotherwise. And, the terms “comprises”, “comprising”, “includes”, and thelike specify the presence of stated features, steps, operations,elements, and/or components but do not preclude the presence or additionof one or more other features, steps, operations, elements, components,and/or groups. Components described as coupled may be electrically ormechanically directly coupled, or they may be indirectly coupled via oneor more intermediate components. All examples and illustrativereferences are non-limiting and should not be used to limit the claimsto specific implementations and embodiments described herein and theirequivalents.

FIG. 1 is a diagrammatic view of a portion of an illustrative kinematicchain 2. Two links 4 a, 4 b are coupled by a revolute joint 6. Forillustrative purposes only, joint 6 is a single degree-of-freedom (DOF)joint. The joint may, however, have two or more DOFs. In addition, thejoint may be a “flexible” component—a continuously bending structure ora “snake-like” mechanism that includes one or more additional smalllinks that function as “vertebrae” as the mechanism bends. It should beunderstood that such “flexible” components may be considered and modeledas a single joint.

A shape sensing optical glass fiber 8 extends between the links 4 a, 4 bthrough joint 6. As shown in detail below, optical fiber 8 extendsthrough a channel that opens out of link 4 a. The fiber may exit thelink essentially on the link's centerline, or the fiber may be offsetfrom the link's centerline. For clarity, most of this description refersto a fiber that exits at the link's centerline, but it should beunderstood that the description refers to a fixture that aligns thefiber in various ways with a link (e.g., through the link on or offcenterline, at the link's outer perimeter, beside the link, and thelike). FIGS. 7A-7D below illustrate fiber/link alignment aspects in moredetail.

Shape sensing optical glass fiber 8 is configured with closely spacedrefractive index change (Bragg) gratings that allow strain to be sensedat multiple locations along the fiber using known methods. In oneinstance the fiber is a three-core fiber with each core configured withsubstantially parallel Bragg gratings approximately 22 mm long andspaced apart by less than ¼ mm. In one instance, approximately 60longitudinally adjacent Bragg gratings are formed for every 20 mm lengthof a core/fiber. Accordingly, the resolution of the sensed strain atvarious fiber locations along the fiber's longitudinal (proximal todistal end) axis is on the order of microns, which allows for accurateshape sensing. This optical fiber description applies to all opticalfibers referred to herein.

If optical fiber 8 is fixed with reference to the links 4 a, 4 b, thenthe length of fiber 8 between the links 4 a, 4 b is constant as thejoint rotates (if the fiber is unconstrained within the joint, the fibermay bow or loop in various ways during the joint rotation). If opticalfiber 8 translates with reference to either or both links 4 a, 4 b, thenthe length of fiber 8 between the links 4 a, 4 b may vary (if the fiberis unconstrained within the joint, the fiber's stiffness may cause it toslide within one or both links when the joint bends). Optical fiber 8may be fixed in orientation with reference to the links 4 a, 4 b.Alternately, the fiber may roll. Using known shape sensing technology,which may be combined with known inverse kinematic calculations by anelectronic data processing system, the position and orientation of link4 b may be determined with reference to link 4 a.

As the joint rotates, as illustrated by alternate position 10 shown inFIG. 1, link 4 b changes in orientation with reference to link 4 a by anangle θ. Accordingly, optical fiber 8 bends, and the fiber bend isexpressed in terms of the fiber's bend radius. The bend radius may varyconsiderably over the length of the bend in order to fit the mechanicsof a particular joint or set of joints. At one or more fiber locationsbetween the links, a minimum fiber bend radius occurs at each jointangle θ.

It has been discovered that for known ways of combining a link and anoptical fiber, that as the joint moves and the fiber bends, a peakstrain occurs in the fiber at a location 12 where the fiber exits thelink. This peak strain saturates the strain interrogation equipment, acondition which causes a loss in the shape sensor's accuracy and updatespeed. For purposes of this disclosure, this situation may be termed alocalized strain saturation problem. FIGS. 2A-2C illustrate thissituation in more detail.

FIG. 2A is a diagrammatic cross-sectional view of fixture portion 14 ofa link in a kinematic chain. Fixture portion 14 supports a shape sensingoptical fiber as the fiber exits the link into a joint region of thekinematic chain. A channel 16 is defined in the fixture portion, and thechannel opens to the joint region at an opening 18. The channel 16 has achannel wall 20. A lip 22 exists between the channel wall 20 and the end24 of fixture portion 14. For example, fixture portion 14 isillustrative of a Newport Corporation Delrin-jawed fiber chuck (part no.FPH-DJ).

FIG. 2B is a diagrammatic cross-sectional view of a shape-sensingoptical fiber 26 mounted in fixture portion 14. Optical fiber 26 extendsthrough channel 16 and exits fixture portion 14 via opening 18. A firstportion 26 a of the optical fiber is positioned within fixture portion14 of a link in the kinematic chain. A second portion 26 b of theoptical fiber extends beyond the link in the kinematic chain and isillustrative of the portion of a shape-sensing optical fiber thatextends into or through a joint in the kinematic chain. Both portions 26a and 26 b are in the portion of the optical fiber that is beinginterrogated for strain information in a shape-sensing system. Asportion 26 b bends as the joint rotates, the optical fiber 26experiences a peak strain at a location 26 c, adjacent lip 22. Thedotted line 28 represents the fiber Bragg gratings (FBG's) in thefiber's cores that are interrogated for strain measurement. Accordingly,a length of interrogated FBG's that experience the peak strain isadjacent lip 22. The term fiber Bragg grating should be broadlyconstrued to include the many variations of such refractive index changegratings.

FIG. 2C is an illustrative diagrammatic cross-sectional view of anothermounting assembly for a shape-sensing optical fiber. As shown in FIG.2C, shape-sensing optical fiber 30 is surrounded by Teflon® FEP tube 32.In one implementation, tube 32 has an approximately 0.4 mm innerdiameter and 1.5 mm outer diameter. Tube 32 is then mounted in fixtureportion 34 in a manner similar to the way fiber 26 is mounted in fixtureportion 14, as shown in FIG. 2B. Since Teflon® FEP is a relatively softmaterial, it was thought that the Teflon® FEP tube 32 would cushionoptical fiber 30 against lip 36 as the fiber and tube were bent duringjoint movement. It was discovered, however, that as the bend radius ofthe tube and fiber decreases, the reactive force from lip 34 istransmitted through the Teflon® FEP tube 32 to the fiber 30, which againcauses a localized peak strain in fiber 30 in the length of FBG's 38 inthe vicinity of region 30 a, which is roughly adjacent to lip 36. Onceagain, the localized strain saturation problem occurs.

The inventors have discovered how to effectively eliminate the region ofpeak strain in the length of shape sensing FGB's adjacent the channellip where the optical fiber exits the fixture for all fiber bend radiidown to the minimum allowed by the material constraints of the fiber.

FIG. 3A is a cross-sectional diagrammatic view of an optical fiberfixture portion 40 in accordance with an aspect of the invention.Fixture portion may be at the end of a link in a kinematic chain. Onenon-limiting example of such a kinematic chain is a teleroboticallycontrolled minimally invasive surgical instrument, such as those used inthe da Vinci® Surgical System manufactured by Intuitive Surgical, Inc.of Sunnyvale, Calif. Fixture portion 40 may be integral with thematerial used to form the link, or it may be one or more separatedevices that are coupled to the link. Non-limiting examples of suchdevices are various collets, chucks, and split clamp mechanisms that maybe used to hold or constrain an optical fiber. Aspects of the inventionas described herein apply to any and all such ways to hold/constrain anoptical fiber.

As shown in FIG. 3A, a channel 42 is defined in fixture portion 40. Incontrast to FIGS. 2A-2C, the lip 44 at the opening 46 where the opticalfiber exits the fixture portion is curved. There are severalcharacteristics to the curve of lip 44. First, the curve beginssubstantially tangent to channel wall 48 (small fabrication anomaliestypically prevent making a geometrically perfect tangential transition,but known fabrication methods, e.g. electrical discharge machining witha shaped electrode, allow such a transition to be substantially andeffectively achieved). Second, the maximum radius of curvature for lip44 is less than the minimum bend radius that the shape-sensing opticalfiber will experience—the bend that the fiber experiences as the jointmoves to its maximum allowable angle θ_(MAX)—so that the fiber remainstangent to the curve of the lip as the fiber bends through its fullrange of motion. This maximum radius of curvature limitation for the lipapplies everywhere on the curved lip surface that substantiallyintersects a bending plane of the fiber (again, known fabricationmethods can be used to effectively create the curved surface, despitesmall fabrication anomalies that may occur). Further, the radius ofcurvature of the lip surface need not be constant. And, in addition, thelip's radius of curvature should not be so small as to effectivelycreate an abrupt edge, which causes localized strain in the fiber asdiscussed above. As a guide for fabrication, a minimum radius ofcurvature for lip 44 may be considered to be approximately ¼ to ½ thefiber's minimum bend radius at the joint's θ_(MAX), although a smallerbend radius is likely acceptable in many applications.

It should be understood that many types of joints exist, and that theminimum bend radius referred to herein is the minimum bend radius of theportion of the fiber in free space immediately adjacent the link fromwhich the fiber extends. In a “snake-like” joint referred to above, forexample, the minimum bend radius of the fiber that is used to determinethe radius of curvature for the curved surface of the lip would be inthe portion of fiber that extends between the link and the first“vertebra” link in the joint.

For single-DOF joint implementations, the curve of the lip need only becoplanar with the plane in which the fiber bends. In otherimplementations, including single- and multi-DOF joints that allowthree-dimensional (3D) position and orientation changes, such as for amulti-DOF ball and socket joint or for a shape sensing optical fiberthat spans multiple single-DOF joints, the lip may be formed inthree-dimensions in a “trumpet-like” shape. In some implementations, the3D shape may be tailored to fit the range of motion of the joint in eachdirection which it is allowed to move, creating shapes that areellipsoid or multiply fluted in cross-section to allow for the resultingfiber bend directions. Further, the channel may have various shapes,such as a cylinder that closely fits around the fiber (either (i) notpermitting the fiber to slide, or (ii) permitting the fiber to slideonly in a longitudinal direction with reference to the link and to rollor twist within the channel) or a slit (e.g., effectively the width ofthe fiber's outer diameter) that permits the fiber to move within theplane of the slit. Various low friction materials and coatings (e.g.,Teflon® FEP tube, Teflon® coating) may be used to facilitate fibersliding within the channel.

FIG. 3B is a cross-sectional diagrammatic view of an illustrativeimplementation of a shape sensing optical fiber 50 mounted in fixtureportion 40 in accordance with aspects of the invention. As shown in FIG.3B, as fiber 50 bends (shown by the dashed lines) with joint rotation,the fiber remains essentially tangent to the curved lip 44 at thechannel's opening (lip 44 is shown curved on both sides of the channelin anticipation of the fiber bending in both directions, but need becurved only in the direction the fiber will bend in a particularembodiment). The region of peak strain is effectively eliminated in thelength of FBG's 52 that is in the vicinity of the curved lip. Thislength of FBG's is a portion of the FBG's that are interrogated forstrain information used to determine fiber shape based on the associatedjoint movement.

Although the implementation illustrated by FIG. 3B is effective, it maybe difficult to create the long, small diameter due to, e.g., drilllimitations. Accordingly, a similar fixture may be made as depicted inFIG. 4. FIG. 4 is another illustrative cross-sectional diagrammatic viewof a portion of an optical fiber fixture 60 in accordance with an aspectof the invention. In the exemplary implementation shown in FIG. 4, thefixture portion includes two pieces. A relatively larger channel iscreated in a first piece 60 a, and then a second piece 60 b having apreformed, relatively smaller channel is inserted into the channel inpiece 60 a. An optical glass fiber 62 is positioned within therelatively smaller channel in second piece 60 b. This aspect allows alarger channel to be formed in piece 60 a using conventional methods(e.g., drilling), and then a tube having the required inner diameter forthe fiber can be inserted as piece 60 b into the channel in piece 60 a.The curved lip 64 where the fiber channel leaves the fixture portion 60and enters the joint region is formed in the second piece.

In an illustrative test assembly implementation (see e.g., FIG. 8), afirst fixture piece 60 a was formed using a fused deposition modeling(FDM) process, and a PEEK tube was used as the second fixture piece 60b. The inner diameter of the PEEK tube was approximately 0.016-inch, sothat a shape sensing optical glass fiber could be held within the tube60 b. The curved, tangential lip 64 was formed using a radius cuttingcenter drill having an approximately 1/64-inch tip diameter and anapproximately 1/16-inch radius curved shoulder.

FIG. 5 is another cross-sectional diagrammatic view of a portion of afixture for a shape sensing optical fiber. As shown in FIG. 5, ashape-sensing optical fiber 70 is positioned inside a flexible tube 70,and the combined fiber 70 and tube 72 is positioned in the portion ofthe fixture 74. Similar to the aspects shown and described above, thelip 76 of the channel in which the combined fiber and tube is positionedis curved. The curve of lip 76 is determined from the minimum bendradius of the fiber as described above, not from the minimum bend radiusof the flexible tube 72. In one non-limiting, illustrativeimplementation the flexible tube 72 is a Teflon® FEP tube as describedabove. The use of a Teflon® FEP tube may be beneficial in certainimplementations, such as those in which the fiber translates along alongitudinal axis of a link as a joint bends due to, e.g., a reaction tochanges in length of the minimum energy fiber path in free space as thejoint bends, or the fiber being offset from the link's centerline.Additional benefits of using a protective tube around the fiber includeprotection for soft coatings, such as acrylate, on the fiber.

Skilled artisans will understand that FIGS. 3A, 3B, 4, and 5 areillustrative of various ways of holding an optical fiber. Curved lipaspects of the invention may be applied to, e.g., various fiber colletand chuck mechanisms, split clamp mechanisms, grooves with the fiberpositioned therein, etc. Skilled artisans will also understand thatvarious aspects as shown and described herein may be combined. As anon-limiting example, a two-piece portion of the fixture assembly asillustrated by FIG. 4 may be used together with the combined flexibletube and fiber as illustrated by FIG. 5.

FIG. 6 is another cross-sectional diagrammatic view of a portion of afixture 80 for a shape sensing optical fiber. In accordance with anotheraspect of the invention, an FBG-configured shape-sensing optical fiber82 is surrounded by a tube 84 of shape memory alloy/superelasticmaterial, such as a Nickel/Titanium alloy. In this aspect of theinvention, the lip 86 of the channel that opens into the joint region isnot curved. Instead, as fiber 82 bends, tube 84 distributes the reactiveforces from lip 86 along the fiber 82 sufficient to avoid producing aregion of local strain in the length of FBG's 88 that is adjacent thelip. In one illustrative, non-limiting implementation, tube 84 is aNitinol tube having a 0.016-inch inner diameter and a 0.026-inch outerdiameter. This Nitinol tube allows for an effective minimum bend radiusfor the fiber in the range of 10-15 mm or less, which is on the order ofhalf that which could be obtained with a non-superelastic material.

FIGS. 7A-7D are diagrammatic views that illustrate various ways that thefixture portion may be positioned with reference to a link. FIG. 7Ashows two links 90 a, 90 b coupled by a revolute joint 92. A shapesensing optical fiber 94 runs between links 90 a and 90 b, through theregion of joint 92. As shown in FIG. 7A, the fixture portion 96 (asdescribed above with reference to FIGS. 3A-6) that holds/constrainsfiber 94 is offset from the centerlines of links 90 a, 90 b (forclarity, fixture portion 96 is indicated only for link 90 a, but mayalso be used for link 90 b if strain interrogation occurs beyond thelocation where fiber 94 enters link 90 b). In some instances, opticalfiber 94 may have to translate (slide) with reference to one or bothlinks as joint 92 rotates. As a non-limiting example, if fiber 92 isfixed in position with reference to link 90 a and allowed to translatewith reference to link 90 b, then fiber 92 may translate in thedirections shown by the arrows as link 90 b moves to alternate positions91 a and 91 b because of the fiber's offset from the link centerlines.Consequently, the length of FBG's adjacent the lip of the channel alsoslides, but the curved lip effectively eliminates peak strain fromoccurring in the FBG's.

FIG. 7B illustrates that a fixture portion may be positioned so that thefiber is generally aligned with an outer perimeter of link 90 a. As anon-limiting example, a groove may be made in an outer surface of thelink, and the shape-sensing fiber may then be laid into the groove andaffixed by, e.g., clamping, gluing, etc. As discussed with reference toFIG. 7A, the fiber may be allowed to translate with reference to eitheror both links 90 a, 90 b as the joint rotates.

FIG. 7C illustrates that a fixture portion may be separate from link 90a and mechanically coupled in any of various conventional ways.

FIG. 7D illustrates that aspects of the invention are not confined touse with single-DOF revolute joints. As a non-limiting example, links 90a and 90 b may be coupled by a prismatic joint 98, and shape-sensingoptical fiber 94 is looped in the joint region to accommodate relativetranslation (i.e., surge) between links 90 a and 90 b, as indicated bythe double-headed arrow. Various other loop shapes (e.g., omega-shaped)may be used.

FIG. 8 is a diagrammatic front elevation view of a test apparatusconstructed in accordance with aspects of the invention and which isillustrative of an optical fiber shape sensing system used in akinematic chain. A first bracket 100 a is fixed to a test bench (notshown), and a second bracket 100 b is coupled to bracket 100 a at asingle-DOF revolute joint 102. Bracket 100 b is coupled to servomotor104, which moves bracket 100 b with reference to bracket 100 a in aconventional manner.

A fiber Bragg grating-configured, three-core optical glass fiber 106 isused in a shape sensing system for the test apparatus. At the proximalend of fiber 106, each core is coupled to a strain interrogation unit108, which is used in the process of determining the fiber's shape, andconsequently the shape of an associated kinematic chain in accordancewith known methods. Such methods include optical time domainreflectometry and optical frequency domain reflectometry as described inU.S. Pat. No. 5,798,521 (filed 27 Feb. 1997) and U.S. Pat. ApplicationPubl. No. US 2007/0065077 (filed 26 Sep. 2006), both of which areincorporated by reference. In one implementation the interrogation unitused was an “Optical Backscatter Reflectometer” and associated softwaresupplied by Luna Innovations Incorporated, Roanoke, Va.

The fiber 106 passes through an optical stage 108, which is supported bybracket 100 a and which allows adjustments in fiber position (x, y, z)and roll orientation. Optical stage 108 supports a collet (not visible),which in turn holds a fixture 110 as described above. In one testimplementation, the fixture portion 110 was as described above withreference to FIG. 4.

The fiber 106 continues through the joint and is held in another fixture112. The fiber may terminate at fixture 112, in which case it may beheld in a manner similar to that shown and described with reference toFIG. 2B. Alternately, the fiber may continue through bracket 100 b (thesecond link), in which case fixture 112 may be configured in accordancewith aspects of the invention to avoid the localized saturation problemwhere the fiber enters the link.

For the shape memory alloy aspect as describe above with reference toFIG. 6, the test apparatus may be modified so that the shape memoryalloy tube extends between brackets 100 a and 100 b, and there is nocurved lip to the channel defined in the fixture portion 110.

FIG. 9 is a diagrammatic view of a minimally invasive surgicalinstrument, such as various grasping, cutting, scissors, retraction, andthe like instruments available for use with the da Vinci® SurgicalSystem. The surgical instrument functions as a kinematic chain in thesurgical system. Details of such instruments are available in variousUnited States Patents. Briefly, a transmission mechanism 120 receivesmechanical forces for telerobotically controlled servomotor actuators.The transmission mechanism 120 transmits the mechanical forces through ahollow instrument shaft 122 via cables and/or cable/hypotube assembliesto actuate both a wrist mechanism 124 and an end effector 126 at thedistal end of the instrument. The wrist mechanism may be a clevis-typemechanism, or it may be a snake-like mechanism. Examples of such wristmechanisms may be found in U.S. Pat. Nos. 6,394,998 (filed 17 Sep. 1999)and 6,817,974 (filed 28 Jun. 2002), both of which are incorporatedherein by reference. In accordance with aspects of the invention, ashape sensing optical fiber 128 (shown in dashed line) is routed throughor along instrument shaft 122 and wrist mechanism 124 (which may bemodeled as one or more joints) to terminate at distal link 130 or endeffector 126. As described above, the shape of optical fiber 128 may bedetermined from sensed strain information and instrument's kinematicpose may be then determined using inverse kinematic calculations. FIG. 9is further illustrative of other instruments and devices that may beused during surgery, such as the various rigid-link and flexible devicesillustrated in U.S. Pat. Application Publ. No. 2008/0065105 (filed 13Jun. 2007), which is incorporated herein by reference.

1-23. (canceled)
 24. An apparatus comprising: a first link, a secondlink, and a joint coupled between the first and second links; a shapememory alloy tube extending between the first link and the second link;and a shape-sensing optical fiber extending through the shape memoryalloy tube, wherein the optical fiber comprises a length of fibercontaining fiber Bragg gratings positioned where the shape memory alloytube extends from the first link.
 25. The apparatus of claim 24: whereinthe shape memory alloy comprises a Nickel/Titanium alloy.
 26. A methodcomprising: assembling a first link, a second link, and a joint coupledbetween the first and second links; extending a shape memory alloy tubebetween the first link and the second link; and positioning ashape-sensing optical fiber through the shape memory alloy tube, whereinthe optical fiber comprises a length of fiber Bragg gratings positionedwhere the shape memory alloy tube extends from the first link.
 27. Themethod of claim 26: wherein the shape memory alloy comprises aNickel/Titanium alloy.
 28. The method of claim 26 further comprising:constraining the optical fiber to slide longitudinally with reference tothe first link.
 29. The method of claim 26 further comprising:constraining the optical fiber to be fixed with reference to the firstlink.
 30. The apparatus of claim 24: wherein the shape memory alloy tubeprovides an effective minimum bend radius for the shape-sensing opticalfiber of less than about 10 mm.
 31. The method of claim 26: whereinpositioning the shape-sensing optical fiber through the shape memoryalloy tube provides an effective minimum bend radius for theshape-sensing optical fiber of less than about 10 mm.
 32. An apparatuscomprising: a first link, a second link, and a joint coupled between thefirst and second links; a tube extending between the first link and thesecond link; and a shape-sensing optical fiber extending through thetube, wherein the optical fiber comprises a length of fiber containingfiber Bragg gratings positioned where the tube extends from the firstlink, and wherein as the optical fiber bends, the tube distributes alocal external force along the optical fiber sufficient to avoidproducing a region of local strain in the length of fiber containingfiber Bragg gratings.
 33. The apparatus of claim 32: wherein the tubecomprises a shape memory alloy.
 34. The apparatus of claim 33: whereinthe shape memory alloy comprises a Nickel/Titanium alloy.
 35. Theapparatus of claim 32: wherein the tube provides an effective minimumbend radius for the shape-sensing optical fiber of less than about 10mm.
 36. The apparatus of claim 32: further comprising a fixture by whichthe optical fiber is constrained with reference to the first link;wherein the fixture comprises a channel defined therein in which thetube is positioned to place the length of fiber containing fiber Bragggratings adjacent a lip of the channel where the channel opens out ofthe fixture; and wherein as the optical fiber bends, the tubedistributes a force from the lip along the optical fiber so as to avoidproducing a region of local strain in the length of fiber containingfiber Bragg gratings that is adjacent the lip.
 37. A method comprising:articulating a first link and a second link relative to each other via ajoint coupled between the first and second links; sensing a position ofthe first link and the second link relative to each other via ashape-sensing optical fiber that extends between the first link and thesecond link; and during bending of the optical fiber, distributing alocal external force along the optical fiber via a tube surrounding theoptical fiber; wherein the optical fiber comprises a length of fibercontaining fiber Bragg gratings positioned where the tube extends fromthe first link; and wherein distributing the local external forcecomprises distributing the local external force along the optical fiberto avoid producing a region of local strain in the length of fibercontaining the fiber Bragg gratings.
 38. The method of claim 37: whereinthe tube comprises a shape memory alloy.
 39. The method of claim 37:wherein the shape memory alloy comprises a Nickel/Titanium alloy. 40.The method of claim 37 further comprising: constraining the opticalfiber to slide longitudinally relative to the first link.
 41. The methodof claim 37 further comprising: constraining the optical fiber to befixed relative to the first link.